Syria's opposition demanded on Thursday that UN chemical weapons inspectors immediately investigate a besieged rebel-held region hit by an alleged chemical weapons attack that killed hundreds of people a day earlier.

Children, affected by what activists say was a gas attack, breathe through oxygen masks in the Damascus suburb of Saqba, August 21, 2013. Credit: Reuters

Syria's opposition demanded on Thursday that United Nations chemical weapons inspectors immediately investigate a besieged rebel-held region hit by an alleged chemical weapons attack that killed hundreds of people a day earlier.

In Paris, French Foreign Minister Laurent Fabius said the international community needed to respond with force if the allegations that the Syrian government was responsible for a chemical attack on civilians proved true, although there was no question of sending troops on the ground.

Syrian President Bashar al-Assad's forces have continued a heavy bombardment of the ring of rebel-held suburbs around the capital, known as the Ghouta region, which activists say will further hinder U.N. investigators from entering the area, only a few kilometers from where the team's Damascus hotel.

"We are asking for this team to go directly, with complete freedom ... to the site of the crimes which took place yesterday," George Sabra, a prominent member of the umbrella opposition's National Coalition, told Reuters.

He said the U.N. Security Council should amend the mission of the team, tasked with investigating a few sites of previous alleged chemical attacks, to give it the right to visit any site.

"But we are doubtful because the mission of these experts is constrained by the Syrian regime, limited to a few areas which it will take them to," he said by telephone.

With Wednesday's death toll estimated between 500 and 1,300, what would be the world's most lethal chemical weapons attack since the 1980s prompted an emergency meeting of the U.N. Security Council in New York. Syrian authorities have denied the army used chemical weapons.

Opposition activists said men, women and children were killed as they slept. Activists say several towns in Ghouta where hit with rockets loaded with poison gas at dawn on Wednesday.

The Security Council did not explicitly demand a U.N. investigation of the incident, although it said "clarity" was needed and welcomed U.N. chief Ban Ki-moon's calls for a prompt investigation by the inspection team in Syria, led by Ake Sellstrom.

An earlier Western-drafted statement submitted to the Security Council, seen by Reuters, was not approved. The final version of the statement was watered down to accommodate objections from Russia and China, diplomats said. Moscow and Beijing have vetoed previous Western efforts to impose U.N. penalties on Assad.

In Paris, Fabius called for action if the allegations proved true. "There would have to be reaction with force in Syria from the international community, but there is no question of sending troops on the ground," he told the BFM television network.

If the Security Council could not make a decision, one would have to be taken "in other ways", he said, without elaborating.

Many rebels and activists in the opposition area say they had lost interest in U.N. investigations or help from Western powers abroad. Some say the rebels should take matters into their own hands and retaliate.

"The families of Ghouta have lost hope in any investigation committees, which have offered us no relief since the revolution began two years ago ... We are 7 kilometers away, just a 5 minute car ride from where they are staying. We're being exterminated with poison gas while they drink their coffee and sit inside their hotels," said activist Bara Abdelraman, speaking by Skype.

"As leaders of the activists and opposition, of course we still call for the entrance of investigators and vow to protect them, as it is a responsibility before God to do everything we can for our people who are being massacred."

REBELS MULL RETALIATION

The Ghouta area is an expanse of farmland dotted with large built-up areas inhabited mostly by members of Syria's Sunni Muslim majority that have been at the forefront of the uprising against Assad's Alawite rule.

Assad's Shi'ite backer Iran said the Syrian government could not have been behind the possible chemical weapon attack as Assad had the upper hand in the fighting.

A report by the opposition al-Sham Research Center said the use of chemical weapons on a scale unseen since their use was first reported last year is "a message" from Assad to Turkey and the Arab Sunni backers of the revolt.

They appeared to have increased their support for the armed opposition, and the attack showed that Assad was not afraid of escalating the conflict, unleashing a new wave or refugees and destabilizing the region, the center said.

Qassem Saadedine, a commander and spokesman for the rebels' Supreme Military Council, said the group was still deliberating on how or if it should respond to the attack.

"We have full sympathy for the people who are becoming desperate as they watch another round of political statements and U.N. meetings without any hope of action, and we are still studying how the rebels should respond," he told Reuters by telephone.

In Ghouta on Thursday, rockets and heavy mortar rounds hit the rebel-held areas of Jobar and Zamalka, which are on the eastern outskirts of the capital, activists said.

Rockets also hit the nearby district of Qaboun to the north, where rebel fighters have repelled attempts by loyalist forces to overrun the area, and the Yarmouk Palestinian refugee camp area to the south, the activists added.

Syrian Information Minister Omran Zoabi said the allegations of chemical attacks, just days after the arrival of a U.N. investigation team, were "illogical and fabricated."

Assad's officials have said they would never use poison gas against Syrians. The United States and European allies believe Assad's forces have used small amounts of sarin before, hence the current U.N. visit.

LIMITED INTERNATIONAL RESPONSE

Immediate international action is likely to be limited, with the divisions among major powers that have crippled efforts to quell 2-1/2 years of civil war still much in evidence.

Russia backed up Syrian government denials by saying it looked like a rebel "provocation" to discredit Assad.

France, Britain, the United States and others called for an immediate on-site investigation by U.N. chemical weapons inspectors who arrived in the Syrian capital only this week. Moscow, urging an "objective" inquiry, said the very presence of that team suggested government forces were not to blame.

U.S. President Barack Obama has made the use of chemical weapons by Assad's forces a "red line" that in June triggered more U.S. aid to the rebels. But previous, smaller and disputed cases of their deployment have not brought the all-out military intervention rebel leaders have sought to break a stalemate.

U.S. Senator John McCain, a Republican critic of Obama's Syria policy, said on Twitter that failure to penalize previous gas attacks had emboldened Assad: "No consequence for Assad using chemical weapons & crossing red line," he said. "We shouldn't be surprised he's using them again."

Images, including some by freelance photographers supplied to Reuters, showed scores of bodies - some of them small children - laid out on floors with no visible signs of external injury. Some showed people with foam around their mouths.

The United States and others said it had no independent confirmation that chemical weapons had been used. The U.N. chief, Ban, said the head of the inspection team in Damascus was already discussing the latest claims with the government.

"SLEEPING DEAD"

In 1988, 3,000 to 5,000 Iraqi Kurds were gassed by Saddam Hussein's forces at Halabja.

The Syrian Network for Human Rights, an opposition monitoring group, said that 587 people were killed by chemical weapons and 78 by conventional shelling, but the organization warned that the death toll was still "initial".

The opposition Syrian National Coalition said 650 people died. One man who said he had retrieved victims in the suburb of Erbin told Reuters: "We would go into a house and everything was in its place. Every person was in their place. They were lying where they had been. They looked like they were asleep."

Doctors interviewed described symptoms they believe point to sarin gas, one of the agents Western powers accuse Damascus of having in an undeclared chemical weapons stockpile.

Extensive amateur video and photographs appeared on the Internet showing victims choking, some foaming at the mouth.

Syria is one of just a handful of countries that are not parties to the international treaty that bans chemical weapons, and Western nations believe it has caches of undeclared mustard gas, sarin and VX nerve agents.